3D Printing for Enhanced On-Chip Stimulated Brillouin Scattering

3D printing has emerged as a big technological transformation in integrated photonics mainly due to its unprecedent design flexibility and rapid prototyping capabilities. Among the various 3D printing techniques, two-photon polymerization (2PP) enables a remarkable printing resolution with feature sizes of down to 100 nm [1], [2]. For 2PP the focal point of an ultrashort pulse laser with an emission wavelength in the near-infrared (NIR) is scanned through a droplet of photosensitive liquid resin, see Fig. 1(a). Polymerization is initiated upon nonlinear absorption of two photons simultaneously, resulting in solidification of the liquid resin in a small region (called voxel). The high precision to fabricate complex 3D polymer nanoscale structures is enabled by the low voxel diameter. The exploitation of this method has been unexplored yet for on-chip stimulated Brillouin scattering (SBS), which is very advantageous in many different fields like microwave photonics (MWP), THz generation [3], filters [4], spectroscopy [5] etc. Several material platforms can be used for on-chip SBS applications, among which chalcogenide (As<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>S<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>) offers the highest gain [6]. However, many of these platforms have their drawbacks like the need for suspending the waveguides, toxicity, or CMOS co-integration issues. In this work, we have fabricated a chip-scale rib waveguide structure via 2PP onto a fused silica substrate as shown in Fig. 1(b). The core material is IP-Dip (Nanoscribe) [7] with cross-sectional geometry of 1934×2200×418×1980 (L1×L2×L3×L4 in nm), see Fig. 1(a). This configuration satisfies the waveguiding of the optical and acoustic modes by total internal reflection. The COMSOL simulation results for SBS in this structure are shown in Fig. 1(c) and (d), demonstrating that it can support the SBS process, resulting in Brillouin gain spectra (Fig. 1(d)). A way to increase the SBS gain is to reduce the size of the waveguide. For sufficiently small structures the SBS gain may increase due to very strong boundary forces (radiation pressure) which may interact with the electrostriction in the bulk of the waveguide if both are in phase. Given the capability of 3D printing to fabricate nanoscale structures [1], [2], it paves the way to on-chip SBS gain that can be higher than for state-of-the-art platforms.